They have a ton of different motors/servos to choose from and I'm a bit overwhelmed.

I'm assuming whatever I get I want it to be geared down maybe 30:1 or 60:1 so that I have ample torque but it can also drive fast enough so that if a big correction is needed, the robot can drive hard in the correction direction. Too high of a gearing and my top speed may not be enough to overcome a large tilt angle.

Do I need a motor controller? I have built a line follower which uses PWM from a microcontroller through two MOSFETS to drive the motors. I haven't constructed an H bridge so the motors can't turn backwards (not necessary for a line follower), but I can't see it being that much more difficult. So I would think I should be able to make my own motor controller for my balancing robot. I like to do as much as I can myself for the learning aspect of things.

That is all a motor controller really does anyways right? A few MOSFETS and an H bridge to drive the motors in either direction.

After much research, I have figured out everything I need except for the motors.

What gyro / accelerometer have you decided for? I'm just curious...

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From what I've seen, most servos come with encoders, which I will probably find useful in a balancing robot, but the servos need to be modified for continuous rotation.

Servos have an encoder (the pot) that you'll deactivate in the process of modifying them for continuous rotation. So an modified servo is actually an DC moter with built-in motor controller and speed control - but no encoders. They're also not very fast: An "fast" Futaba servo has the equivalent of 111RPM (calculated based on time to rotate 60 degree).

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I'm assuming whatever I get I want it to be geared down maybe 30:1 or 60:1 so that I have ample torque but it can also drive fast enough so that if a big correction is needed, the robot can drive hard in the correction direction. Too high of a gearing and my top speed may not be enough to overcome a large tilt angle.

I'm confused. The websites quote the RPM and the torque for gearhead motors. Is that RPM and torque BEFORE applying the specified reduction? Or is it RPM before reduction and torque after reduction (so the numbers look better )

The Devantech RD01 propulsion kit is a complete kit for any robot and it will work with a balancing one. Actually, it uses EMG30 motors that someone from UK was using to build a balancing robot with Arduino and gyro + accelerometer. The motors are great, but they have a bit of backlash that you can hear clicking during balancing. Balancing robots need high precision gear reduction with no backlash. Perhaps a higher speed motor (less gearing) and a toothed belt reduction for the last stage will work best. Also, it is a good idea to get motors with built in encoders. I am having difficulties to build efficient encoders since I don't have a machine shop. Also it is a good idea to get a ready made motor controller/driver. I know it can be self made, but balancing robots need high quality parts. Anything can throw in a little error here and there so the robot doesn't stay balanced well.

An ADXL202JE accelerometer (Dual axis 2g) and an SFE single axis gyro, either 75 or 150 degree/second. I would assume 75 deg/sec would give me better resolution on a balancing bot, but I can probably get away with 150 deg/sec, which would then allow me to reuse that gyro later in another application. I'm undecided on this.

The Devantech RD01 propulsion kit is a complete kit for any robot and it will work with a balancing one. Actually, it uses EMG30 motors that someone from UK was using to build a balancing robot with Arduino and gyro + accelerometer. The motors are great, but they have a bit of backlash that you can hear clicking during balancing. Balancing robots need high precision gear reduction with no backlash. Perhaps a higher speed motor (less gearing) and a toothed belt reduction for the last stage will work best. Also, it is a good idea to get motors with built in encoders. I am having difficulties to build efficient encoders since I don't have a machine shop. Also it is a good idea to get a ready made motor controller/driver. I know it can be self made, but balancing robots need high quality parts. Anything can throw in a little error here and there so the robot doesn't stay balanced well.

I am going to assume that since the EMG30's seem to be a bit better quality than most of the motors for sale on these websites and have noticeable backlash, that pretty much all geared motors for sale on these websites will have comparable backlash. Would that be correct? Or are the EMG30's worse than normal?

I've never bought motors from Lynxmotion because I could not aford it yet. So I can't say anything about them.

As for planetary motrs, they are noisy but very powerfull, also need high current motor drivers. They come from drills or electric screw drivers, so the robot will sound like 2 drills running around. This is not what I have in mind for a robot.

The backlash is very important for a balancing robot because a little backlash in the axle makes the top of the robot freely move a few degrees...

1 degree sounds alright. Someone mentioned before that the EMG30's had noticable backlash as well. What is the best motor for a balancing bot in terms of as little backlash as possible.

I think when I looked at David Anderson's nBot, it said he used EMG30's and the videos of it balancing looked alright.

I know I will probably end up going with a motor already mentioned in this thread, but if there is something else out there that is reasonably priced with little backlash, let me know.

A pulley drive would reduce backlash probably better than a high-priced motor, but I don't really want to deal with belt-drive issues. Plus out of all the balancing robots I have seen, I only recall one or two that used belt drive and they were designed by University labs ($$$).